Superconductive wire, stacked superconductive wire, superconductive coil and superconductive cable

ABSTRACT

A superconductive wire according to an embodiment of the present disclosure includes a first member and a second member. The first member includes a first substrate made of a conductive material, a first intermediate layer made of a conductive material and disposed on the first substrate, and a first superconductive layer made of a superconductive material and disposed on the first intermediate layer. The second member includes a second substrate made of a conductive material, a second intermediate layer made of a conductive material and disposed on the second substrate, and a second superconductive layer made of a superconductive material and disposed on the second intermediate layer. The first member and the second member are stacked along a thickness direction of the superconductive wire so that the first superconductive layer and the second superconductive layer face each other. The first superconductive layer is electrically connected to the second superconductive layer.

TECHNICAL FIELD

The present disclosure relates to a superconductive wire, a stackedsuperconductive wire, a superconductive coil, and a superconductivecable.

BACKGROUND ART

For example, Japanese Patent Laying-Open No. 2012-156048 (PTL 1)describes a conventional superconductive wire.

The superconductive wire described in PTL 1 includes a substrate, anintermediate layer disposed on the substrate, an oxide superconductorlayer disposed on the intermediate layer, and a stabilization layerdisposed on the oxide superconductor layer. The intermediate layer ismade of an insulating material.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laying-Open No. 2012-156048

SUMMARY OF INVENTION

A superconductive wire according to an embodiment of the presentdisclosure includes a first member and a second member. The first memberincludes a first substrate made of a conductive material, a firstintermediate layer made of a conductive material and disposed on thefirst substrate, and a first superconductive layer made of asuperconductive material and disposed on the first intermediate layer.The second member includes a second substrate made of a conductivematerial, a second intermediate layer made of a conductive material anddisposed on the second substrate, and a second superconductive layermade of a superconductive material and disposed on the secondintermediate layer. The first member and the second member are stackedalong a thickness direction of the superconductive wire so that thefirst superconductive layer and the second superconductive layer faceeach other. The first superconductive layer is electrically connected tothe second superconductive layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view illustrating a superconductive wire 10 according toan embodiment;

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1;

FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1;

FIG. 4 is a cross-sectional view taken along the longitudinal directionof a first end 10 a of a superconductive wire 10 according to a firstmodification of the embodiment;

FIG. 5 is a cross-sectional view taken along the longitudinal directionof a first end 10 a of a superconductive wire 10 according to a secondmodification of the embodiment;

FIG. 6 is a cross-sectional view illustrating a superconductive wire 10according to a third modification of the embodiment;

FIG. 7 is a cross-sectional view illustrating a superconductive wire 10according to a fourth modification of the embodiment;

FIG. 8 is a top view illustrating a stacked superconductive wire 20according to an embodiment;

FIG. 9 is a cross-sectional view taken along line IX-IX in FIG. 8;

FIG. 10 is a cross-sectional view illustrating a stacked superconductivewire 20 according to a first modification of the embodiment;

FIG. 11 is a cross-sectional view illustrating a stacked superconductivewire 20 according to a second modification of the embodiment; FIG. 12 isa cross-sectional view illustrating a stacked superconductive wire 20according to a third modification of the embodiment;

FIG. 13 is a perspective view illustrating a superconductive coil 30according to an embodiment;

FIG. 14 is a cross-sectional view perpendicular to a central axis A ofthe superconductive coil 30 according to the embodiment;

FIG. 15 is a side view illustrating a superconductive cable 40 accordingto an embodiment; and

FIG. 16 is a cross-sectional view taken along line XVI-XVI of FIG. 15.

DETAILED DESCRIPTION Problem to be Solved by the Present Disclosure

In the superconductive wire described in PTL 1, when a quench (aphenomenon of transition from the superconductive state to the normalconductive state) occurs due to the local degradation of characteristicsin the oxide superconductor layer, an electric current flowing in theoxide superconductor layer is bypassed to the stabilization layer.

In other words, in the superconductive wire described in PTL 1, there isonly one path to bypass the electric current when a quench occurs.Therefore, in the superconductive wire described in PTL 1, it isrequired to form the stabilization layer thicker so as to ensure theanti-quench resistance, which leads to an increase in cost. Thus, thesuperconductive wire described in PTL 1 has a room for improvement inthe anti-quench resistance.

The present disclosure has been made in view of the above-describedproblems in the prior art. Specifically, the present disclosure providesa superconductive wire capable of improving the anti-quench resistance.

Advantageous Effect of the Present Disclosure

According to the superconductive wire of the present disclosure, theanti-quench resistance is improved.

Description of Aspects of the Present Disclosure

First, a description will be given on each aspect of the presentdisclosure.

(1) A superconductive wire according to an aspect of the presentdisclosure includes a first member and a second member. The first memberincludes a first substrate made of a conductive material, a firstintermediate layer made of a conductive material and disposed on thefirst substrate, and a first superconductive layer made of asuperconductive material and disposed on the first intermediate layer.The second member includes a second substrate made of a conductivematerial, a second intermediate layer made of a conductive material anddisposed on the second substrate, and a second superconductive layermade of a superconductive material and disposed on the secondintermediate layer. The first member and the second member are stackedalong a thickness direction of the superconductive wire so that thefirst superconductive layer and the second superconductive layer faceeach other. The first superconductive layer is electrically connected tothe second superconductive layer.

In the superconductive wire described above in (1), when a quench occursin the first superconductive layer (or the second superconductivelayer), an electric current flowing in the first superconductive layer(or the second superconductive layer) flows in the secondsuperconductive layer (or the first superconductive layer) and the firstsubstrate (or the second substrate). In other words, in thesuperconductive wire of the above (1), there are two paths to bypass theelectric current when a quench occurs. Therefore, according to thesuperconductive wire described above in (1), it is possible to improvethe anti-quench resistance. In the superconductive wire described abovein (1), the first superconductive layer and the second superconductivelayer are arranged close to the neutral line of the superconductivewire. Therefore, according to the superconductive wire described abovein (1), it is possible to reduce the bending stress acting on the firstsuperconductive layer and the second superconductive layer when thesuperconductive wire is being bent.

(2) In the superconductive wire according to (1), the firstsuperconductive layer may be superconductively bonded to the secondsuperconductive layer.

According to the superconductive wire described above in (2), when aquench occurs in the first superconductive layer (or the secondsuperconductive layer), the electric current flowing in the firstsuperconductive layer (or the second superconductive layer) may bebypassed to the second superconductive layer (or the firstsuperconductive layer) without passing through the normal conductor.Further, according to the superconductive wire described above in (2),since the thickness of the superconductive layer is substantiallyincreased by superconductively bonding the first superconductive layerto the second superconductive layer, the local degradation ofcharacteristics is less likely to occur in the superconductive layer.

(3) In the superconductive wire according to (1), the first member mayfurther include a first protective layer made of silver or a silveralloy and disposed on the first superconductive layer, the second membermay further include a second protective layer made of silver or a silveralloy and disposed on the second superconductive layer, and the firstprotective layer may be directly bonded to the second protective layer.

According to the superconductive wire described above in (3), when aquench occurs in the first superconductive layer (or the secondsuperconductive layer), the electric current flowing in the firstsuperconductive layer (or the second superconductive layer) may bebypassed to the second superconductive layer (or the firstsuperconductive layer) without passing through a material having ahigher electric resistance.

(4) The superconductive wire described above in (1) may further includea bonding layer, the first member may further include a first protectivelayer made of a conductive material and disposed on the firstsuperconductive layer, the second member may further include a secondprotective layer made of a conductive material and disposed on thesecond superconductive layer, and the first protective layer may bebonded to the second protective layer via the bonding layer.

In the superconductive wire described above in (4), since there are twopaths to bypass the electric current when a quench occurs, theanti-quench resistance is improved.

(5) In the superconductive wire according to (4), the first protectivelayer and the second protective layer may be made of silver or a silveralloy, and the bonding layer may be made of solder.

According to the superconductive wire described above in (5) above, itis possible to improve the bonding between the first member and thesecond member by the bonding layer while improving the anti-quenchresistance.

(6) In the superconductive wire according to (4) or (5), the bondinglayer may be exposed at an end of the superconductive wire in thelongitudinal direction by removing either the first member or the secondmember therefrom.

According to the superconductive wire described above in (6), it ispossible to connect the superconductive wire to an external power sourcevia the bonding layer.

(7) In the superconductive wire according to any one of (4) to (6), thefirst protective layer and the second protective layer each may have athickness of 5 μm or less.

Since the first protective layer (or the second protective layer) isonly used to bypass the electric current to the second superconductivelayer (or the first superconductive layer) when a quench occurs in thefirst superconductive layer (or the second superconductive layer), evenif the first protective layer and the second protective layer are formedto be relatively thin, the influence on the anti-quench resistance issmall. On the other hand, since the first protective layer and thesecond protective layer are formed to be relatively thin, the firstsuperconductive layer and the second superconductive layer may bearranged closer to the neutral line of the superconductive wire, whichmakes it possible to further reduce the bending stress acting on thefirst superconductive layer and the second superconductive layer whenthe superconductive wire is being bent. Thus, according to thesuperconductive wire described above in (7), it is possible to furtherreduce the bending stress acting on the first superconductive layer andthe second superconductive layer while improving the anti-quenchresistance.

(8) In the superconductive wire according to any one of the above (1) to(7), the first substrate may include a first base layer and a firstconductive layer made of a material having a lower electric resistancethan a material of the first base layer and disposed between the firstbase layer and the first intermediate layer, and the second substratemay include a second base layer, and a second conductive layer made of amaterial having a lower electric resistance than a material of thesecond base layer and disposed between the second base layer and thesecond intermediate layer.

As described above, when a quench occurs in the first superconductivelayer (or the second superconductive layer), the electric currentflowing in the first superconductive layer (or the secondsuperconductive layer) is also bypassed to the first substrate (or thesecond substrate), and the bypassed current flows through the firstconductive layer (or the second conductive layer) having a relativelylow electric resistance, which makes it possible for the superconductivewire according to (8) to reduce the electric resistance when bypassingthe electric current.

(9) In the superconductive wire according to (8), the first conductivelayer may be exposed at an end of the superconductive wire in thelongitudinal direction by removing the first base layer therefrom.

Thus, the superconductive wire according to (9) is electricallyconnected to an external power source via the first conductive layerhaving a relatively low electric resistance, and thereby, it is possiblefor the superconductive wire described above in (9) to reduce theconnection resistance to the external power source.

(10) In the superconductive wire according to (8) or (9), the secondconductive layer may be exposed at an end of the superconductive wire inthe longitudinal direction by removing the second base layer therefrom.

Thus, the superconductive wire according to (10) is electricallyconnected to an external power source via the second conductive layerhaving a relatively low electric resistance, and thereby, it is possiblefor the superconductive wire described above in (10) to reduce theconnection resistance to the external power source.

(11) In the superconductive wire according to any one of (8) to (10),the first conductive layer and the second conductive layer may be madeof copper or a copper alloy, and the first substrate and the secondsubstrate may be made of stainless steel or Hastelloy.

According to the superconductive wire described above in (11), it ispossible to reduce the electric resistance when bypassing the electriccurrent.

(12) In the superconductive wire according to the above (1), the firstmember and the second member may be spaced from each other at an end ofthe superconductive wire in the longitudinal direction.

According to the superconductive wire described above in (12), thesuperconductive wire may be connected to an external power source byinserting a lead between the first member and the second member.

(13) A stacked superconductive wire according to an aspect of thepresent disclosure includes a plurality of superconductive wiresdescribed above in any one of (1) to (12). The plurality ofsuperconductive wires are stacked along a thickness direction of thestacked superconductive wire, and a value obtained by dividing athickness of the stacked superconductive wire by a width of the stackedsuperconductive wire in a direction orthogonal to the longitudinaldirection of the stacked superconductive wire is 0.5 or more and 2.0 orless.

According to the stacked superconductive wire described above in (13),since the ratio of the thickness to the width in the directionorthogonal to the longitudinal direction is relatively large, the wireis easily handled.

(14) A superconductive coil according to an aspect of the presentdisclosure includes the superconductive wire according to any one of (1)to (12) and an insulating material. The superconductive wire is woundaround a central axis of the superconductive coil and impregnated withthe insulating material.

Due to the difference in thermal expansion coefficient between thesuperconductive wire and the insulating material, a tensile stress mayact on the superconductive wire in a direction of peeling off thesuperconductive layer. This tensile stress may peel the superconductivelayer off from the superconductive wire. However, in the superconductivewires according to any one of (1) to (12), since the superconductivelayer is sandwiched between the first substrate and the secondsubstrate, the tensile stress is less likely to act on thesuperconductive layer. Therefore, according to the superconductive coil(14), it is possible to prevent the superconductive layer (the firstsuperconductive layer and the second superconductive layer) from beingpeeled off by the tensile stress caused by the difference in thermalexpansion coefficient between the superconductive wire and theinsulating material.

(15) In the superconductive coil according to (14), the superconductivewire may have a lead-out portion drawn out from the superconductive wirewound around the central axis of the superconductive coil, and theminimum radius of curvature of the lead-out portion in thesuperconductive wire may be 20 mm or less.

As described above, in the superconductive wires according to any one of(1) to (12), the first superconductive layer and the secondsuperconductive layer are arranged close to the neutral line of thesuperconductive wire, and thereby, it is possible to reduce the bendingstress acting on the first superconductive layer and the secondsuperconductive layer when the superconductive wire is being bent.Therefore, according to the superconductive coil (15), it is possible toform the superconductive coil so that the minimum radius of curvature ofthe lead-out portion is 20 mm or less.

(16) A superconductive cable according to an aspect of the presentdisclosure includes the superconductive wire according to any one of (1)to (12) and a former. The superconductive wire is spirally wound arounda central axis of the former on the outer peripheral surface of theformer, and the minimum radius of curvature of the superconductive wireis 20 mm or less.

As described above, in the superconductive wires according to any one of(1) to (12), the first superconductive layer and the secondsuperconductive layer are arranged close to the neutral line of thesuperconductive wire, and thereby, it is possible to reduce the bendingstress acting on the first superconductive layer and the secondsuperconductive layer when the superconductive wire is being bent.Therefore, according to the superconductive cable (16), it is possibleto form the superconductive coil so that the minimum radius of curvatureis 20 mm or less.

Details of Embodiments of the Present Disclosure

Details of embodiments of the present disclosure will now be describedwith reference to the drawings. In the following drawings, the same orcorresponding portions are denoted by the same reference numerals, andthe description thereof will not be repeated.

(Configuration of Superconductive Wire)

Hereinafter, the configuration of a superconductive wire 10 according toan embodiment will be described.

FIG. 1 is a top view illustrating a superconductive wire 10 according toan embodiment. As illustrated in FIG. 1, the superconductive wire 10 hasa first end 10 a and a second end 10 b . The first end 10 a is one endof the superconductive wire 10 in the longitudinal direction. The secondend 10 b is the other end of the superconductive wire 10 that isopposite to the first end 10 a.

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. Asillustrated in FIG. 2, the superconductive wire 10 includes a firstmember 11, a second member 12, and a bonding layer 13.

The first member 11 includes a first substrate 11 a, a firstintermediate layer 11 b, a first superconductive layer 11 c, and a firstprotective layer 11 d.

The first substrate 11 a is made of a conductive material. The firstsubstrate 11 a includes a first base layer 11 as and a first conductivelayer 11 ab. The first base layer 11 aa and the first conductive layer11 ab are made of a conductive material. The electric resistance of thefirst conductive layer 11 ab is lower than the electric resistance ofthe first base layer 11 aa. The first conductive layer 11 ab is disposedon the first base layer 11 aa.

The first base layer 11 aa is made of, for example, stainless steel orHastelloy (registered trademark). The first conductive layer 11 ab ismade of, for example, copper (Cu) or a copper alloy. It should be notedthat the material of the first base layer 11 aa and the first conductivelayer 11 ab is not limited thereto.

The first intermediate layer 11 b is disposed on the first substrate 11a. More specifically, the first intermediate layer 11 b is disposed onthe first conductive layer 11 ab. The first intermediate layer 11 b ismade of a conductive material. The first intermediate layer 11 b is madeof, for example, strontium titanate (SrTiO₃) doped with niobium (Nb). Itshould be noted that the material of the first intermediate layer 11 bis not limited thereto.

The first superconductive layer 11 c is made of a superconductor. Thefirst superconductive layer 11 c is made of, for example, an oxidesuperconductor. For example, the oxide superconductor of the firstsuperconductive layer 11 c may be REBaCu₃O_(x) (x is any number of 6 ormore and 8 or less, and RE represents a rare earth element such asyttrium (Y), gadolinium (Gd), samarium (Sm), or holmium (Ho)). The firstsuperconductive layer 11 c is disposed on the first intermediate layer11 b. It should be noted that the material of the first superconductivelayer 11 c is not limited thereto.

As described above, since the first intermediate layer 11 b is made of aconductive material, the first superconductive layer 11 c and the firstsubstrate 11 a (i.e., the first conductive layer 11 ab) are electricallyconnected to each other.

The first protective layer 11 d is disposed to cover the outerperipheral surface of the first member 11. More specifically, the firstprotective layer 11 d is disposed on the upper surface of the firstsuperconductive layer 11 c, the side surfaces of the firstsuperconductive layer 11 c, the side surfaces of the first intermediatelayer 11 b, the side surfaces of the first conductive layer 11 ab, theside surfaces of the first base layer 11 aa, and the bottom surface ofthe first base layer 11 aa. The first protective layer 11 d is made of aconductive material. The first protective layer 11 d is made of, forexample, silver (Ag) or a silver alloy. It should be noted that thematerial of the first protective layer 11 d is not limited thereto. Thethickness T1 of the first protective layer 11 d on the firstsuperconductive layer 11 c is, for example, 10 μm or less. The thicknessT1 is preferably 5 μm or less. The thickness T1 is, for example, 1 μm ormore.

The second member 12 includes a second substrate 12 a, a secondintermediate layer 12 b, a second superconductive layer 12 c, and asecond protective layer 12 d.

The second substrate 12 a is made of a conductive material. The secondsubstrate 12 a includes a second base layer 12 aa and a secondconductive layer 12ab. The second base layer 12 aa and the secondconductive layer 12 ab are made of a conductive material. The electricresistance of the second conductive layer 12 ab is lower than theelectric resistance of the second base layer 12 aa. The secondconductive layer 12 ab is disposed on the second base layer 12 aa. Inthe above example, it is described that the first substrate 11 aincludes the first base layer 11 aa and the first conductive layer 11ab, and the second substrate 12 a includes the second base layer 12 aaand the second conductive layer 12 ab, it is acceptable that at leastone of the first substrate 11 a and the second substrate 12 a is formedby a base layer only.

The second base layer 12 aa is made of, for example, stainless steel orHastelloy (registered trademark). The second conductive layer 12 ab ismade of, for example, copper or a copper alloy. It should be noted thatthe material of the second base layer 12 aa and the second conductivelayer 12 ab is not limited thereto.

The second intermediate layer 12 b is disposed on the second substrate12 a. More specifically, the second intermediate layer 12 b is disposedon the second conductive layer 12 ab. The second intermediate layer 12 bis made of a conductive material. The second intermediate layer 12 b ismade of, for example, strontium titanate doped with niobium. It shouldbe noted that the material of the second intermediate layer 12 b is notlimited thereto.

The second superconductive layer 12 c is made of a superconductor. Thesecond superconductive layer 12 c is made of, for example, an oxidesuperconductor.

The oxide superconductor of the second superconductive layer 12 c is,for example, REBaCu₃O_(x). It should be noted that the material of thesecond superconductive layer 12 c is not limited thereto. The secondsuperconductive layer 12 c is disposed on the second intermediate layer12 b.

As described above, since the second intermediate layer 12 b is made ofa conductive material, the second superconductive layer 12 c and thesecond substrate 12 a (the second conductive layer 12 ab) areelectrically connected to each other.

The second protective layer 12 d is disposed to cover the outerperipheral surface of the second member 12. More specifically, thesecond protective layer 12 d is disposed on the upper surface of thesecond base layer 12 aa, the side surfaces of the second base layer 12aa, the side surfaces of the second conductive layer 12 ab, the sidesurfaces of the second intermediate layer 12 b, the side surfaces of thesecond superconductive layer 12 c, and the bottom surface of the secondsuperconductive layer 12 c. The second protective layer 12 d is made ofa conductive material. The second protective layer 12 d is made of, forexample, silver or a silver alloy. It should be noted that the materialof the second protective layer 12 d is not limited thereto. Thethickness T2 of the second protective layer 12 d on the secondsuperconductive layer 12 c is, for example, 10 μm or less. The thicknessT2 is preferably 5 μm or less. The thickness T2 is, for example, 1 nm ormore.

The first member 11 and the second member 12 are stacked along thethickness direction of the superconductive wire 10 such that the firstsuperconductive layer 11 c and the second superconductive layer 12 cface each other with the first protective layer 11 d and the secondprotective layer 12 d interposed therebetween.

The bonding layer 13 is disposed between the first member 11 and thesecond member 12. Specifically, the bonding layer 13 is disposed betweenthe first protective layer 11 d on the first superconductive layer 11 cand the second protective layer 12 d on the second superconductive layer12 c. The bonding layer 13 bonds together the first protective layer 11d on the first superconductive layer 11 c and the second protectivelayer 12 d on the second superconductive layer 12 c. The bonding layer13 may be disposed so as to reach the side surfaces of the first member11 and the side surfaces of the second member 12. The bonding layer 13is made of a conductive material. The bonding layer 13 is, for example,a solder. The thickness T3 of the bonding layer 13 is, for example, 10μm or less.

As described above, since the first protective layer 11 d, the secondprotective layer 12 d and the bonding layer 13 are made of a conductivematerial, the first superconductive layer 11 c and the secondsuperconductive layer 12 c are electrically connected to each other.

FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1. Asillustrated in FIG. 3, the first conductive layer 11 ab is exposed atthe first end 10 a by removing the first base layer 11 aa therefrom, andthe second conductive layer 12 ab is exposed at the first end 10 a byremoving the second base layer 12 aa therefrom. It is acceptable that atleast one of the first conductive layer 11 ab and the second conductivelayer 12 ab is exposed at the first end 10 a by removing at least one ofthe first base layer 11 aa and the second base layer 12 aa therefrom.The superconductive wire 10 is connected to an external power source viathe first conductive layer 11 ab and the second conductive layer 12 abexposed at the first end 10 a.

Although not shown, at least one of the first conductive layer 11 aa andthe second conductive layer 12 ab may be exposed at the second end 10 bby removing at least one of the first base layer 11 aa and the secondbase layer 12 aa therefrom.

FIG. 4 is a cross-sectional view taken along the longitudinal directionof a first end 10 a of a superconductive wire 10 according to a firstmodification of the embodiment. As illustrated in FIG. 4, the secondmember 12 may be removed from the first end 10 a . Thus, the bondinglayer 13 is exposed at the first end 10 a . It is acceptable that thebonding layer 13 is exposed at the first end 10 a by removing the firstmember 11 from the first end 10 a . Thereby, the superconductive wire 10is connected to an external power source via the bonding layer 13exposed by removing the second member 12 (or the first member 11).Although not shown, it is acceptable that the bonding layer 13 is alsoexposed at the second end 10 b by removing either the first member 11 orthe second member 12 from the second end 10 b.

FIG. 5 is a cross-sectional view taken along the longitudinal directionof a first end 10 a of a superconductive wire 10 according to a secondmodification of the embodiment. As illustrated in FIG. 5, the firstmember 11 and the second member 12 may be spaced from each other at thefirst end 10 a . For example, the first member 11 and the second member12 are spaced from each other by melting the bonding layer 13 at thefirst end 10 a . A lead 15 may be sandwiched between the first member 11and the second member 12 at the first end 10 a . The lead 15 is made of,for example, copper. The superconductive wire 10 is connected to anexternal power source via the lead 15.

FIG. 6 is a cross-sectional view illustrating a superconductive wire 10according to a third modification of the embodiment. As illustrated inFIG. 6, the superconductive wire 10 does not include the bonding layer13. In other words, in the superconductive wire 10, the first protectivelayer 11 d on the first superconductive layer 11 c is directly bonded tothe second protective layer 12 d on the second superconductive layer 12c.

In this case, the first member 11 and the second member 12(specifically, the first protective layer 11 d on the firstsuperconductive layer 11 c and the second protective layer 12 d on thesecond superconductive layer 12 c) are heated to a predeterminedtemperature, and bonded together by pressing. The predeterminedtemperature is, for example, 500° C. or more and 600° C. or less.

FIG. 7 is a cross-sectional view illustrating a superconductive wire 10according to a fourth modification of the embodiment. As illustrated inFIG. 7, the superconductive wire 10 does not include the firstprotective layer 11 d and the second protective layer 12 d, but includesa bonding layer 14 instead of the bonding layer 13. The bonding layer 14is made of the superconductor that is used to form the firstsuperconductive layer 11 c and the second superconductive layer 12 c. Inother words, the first superconductive layer 11 c and the secondsuperconductive layer 12 c are superconductively bonded.

In this case, the superconductive bonding between the firstsuperconductive layer 11 c and the second superconductive layer 12 c isachieved by the following method. Firstly, an organic compound film isformed on one of the first superconductive layer 11 c and the secondsuperconductive layer 12 c. The organic compound film containsconstituent elements of the superconductor that is used to form thebonding layer 14.

Secondly, the organic compound film is pre-calcined. The organiccompound film is converted to a precursor of the superconductor of thebonding layer 14 by the pre-calcination (hereinafter, the organiccompound film subjected to the pre-calcination is referred to as apre-calcined film). The pre-calcination is performed at a temperaturelower than the temperature at which the material of the bonding layer 14is formed. Thirdly, a heat treatment is performed on the pre-calcinedfilm after the pre-calcination. Thereby, carbide contained in thepre-calcined film is decomposed to form a microcrystalline filmcontaining superconductor microcrystals for forming the firstsuperconductive layer 11 c and the second superconductive layer 12 c.

Fourthly, the first member 11 and the second member 12 are heated andpressed in a state in which the first superconductive layer 11 c and thesecond superconductive layer 12 c are stacked so as to face each otherwith the microcrystalline film interposed therebetween. Thus, thesuperconductor microcrystals contained in the microcrystalline film forforming the first superconductive layer 11 c and the secondsuperconductive layer 12 c are epitaxially grown on the firstsuperconductive layer 11 c and the second superconductive layer 12 c,respectively. Thereby, the superconductive bonding between the firstsuperconductive layer 11 c and the second superconductive layer 12 c isachieved.

(Configuration of Stacked Superconductive Wire)

Hereinafter, the configuration of a stacked superconductive wire 20according to an embodiment will be described.

FIG. 8 is a top view illustrating a stacked superconductive wire 20according to an embodiment. FIG. 9 is a cross-sectional view taken alongline IX-IX in FIG. 8. As illustrated in FIGS. 8 and 9, the stackedsuperconductive wire 20 includes a plurality of superconductive wires10. The stacked superconductive wire 20 is obtained by stacking aplurality of superconductive wires 10 along the thickness direction.Although not shown, the superconductive wires 10 are bonded to eachother by using a bonding layer made of solder or the like.

The stacked superconductive wire 20 has a thickness T4 and a width W ina cross section orthogonal to the longitudinal direction. The thicknessT4 is measured at a location where the thickness of the stackedsuperconductive wire 20 is maximum, and the width W is measured at alocation where the width of the stacked superconductive wire 20 ismaximum. The value obtained by dividing the thickness T4 by the width Wis 0.5 or more and 2.0 or less. Preferably, the value obtained bydividing the thickness T4 by the width W is 0.75 or more and 1.25 orless.

If the thickness of the first base layer 11 aa (the second base layer 12aa) is set to about 50 μm to 100 μm, the thickness of the firstconductive layer 11 ab (the second conductive layer 12 ab) is about 10μm to 50 μm, the thickness of the first intermediate layer 11 b (thesecond intermediate layer 12 b) is set to about 0.1 μm to 0.5 μm, thethickness of the first superconductive layer 11 c (the secondsuperconductive layer 12 c) is set to about 2 μm to 4 μm, the thicknessof the first protective layer 11 d (the second protective layer 12 d) isset to about 1 μm to 10 μm, and the thickness of the bonding layer 13 isset to about 10 μm, then the thickness T4 is about 150 μm to 300 μm, andif the width W is set to about 1 mm, when several superconductive wires10 are stacked, the ratio of the thickness T4 to the width W satisfiesthe relationship of 0.5≤T4/W≤2.0.

FIG. 10 is a cross-sectional view illustrating a stacked superconductivewire 20 according to a first modification of the embodiment. Asillustrated in FIG. 10, the cross section orthogonal to the longitudinaldirection of the stacked superconductive wire 20 may have, for example,a circular shape.

FIG. 11 is a cross-sectional view illustrating a stacked superconductivewire 20 according to a second modification of the embodiment. Asillustrated in FIG. 11, the cross section orthogonal to the longitudinaldirection of the stacked superconductive wire 20 may have, for example,a diamond shape.

FIG. 12 is a cross-sectional view illustrating a stacked superconductivewire 20 according to a third modification of the embodiment. Asillustrated in FIG. 12, the cross section orthogonal to the longitudinaldirection of the stacked superconductive wire 20 may have, for example,a regular hexagonal shape.

The cross section perpendicular to the longitudinal direction of thestacked superconductive wire 20 is not limited to those illustrated inFIGS. 10 to 12, and may have a polygonal shape or an elliptical shape.The stacked superconductive wire 20 illustrated in FIGS. 10 to 12 isobtained by stacking a plurality of superconductive wires 10 along thethickness direction and then subjecting the stacked superconductive wireto a machining process such as cutting. In any of the cross-sectionalshapes illustrated in FIGS. 10 to 12, the relationship of 0.5≤T4/W≤2.0is satisfied.

(Configuration of Superconductive Coil)

Hereinafter, the configuration of a superconductive coil 30 according toan embodiment will be described.

FIG. 13 is a perspective view illustrating the superconductive coil 30according to the embodiment. FIG. 14 is a cross-sectional viewperpendicular to a central axis A of the superconductive coil 30according to the embodiment. As illustrated in FIGS. 13 and 14, thesuperconductive coil 30 is, for example, a pancake coil. It should benoted that the superconductive coil 30 is not limited thereto. Thesuperconductive coil 30 may be, for example, a solenoid coil.

The superconductive coil 30 has a central axis A. The superconductivecoil 30 is formed by winding the superconductive wire 10 around thecentral axis A. The superconductive wire 10 is fixed in shape byimpregnating the superconductive wire 10 with an insulating material 31such as epoxy resin after being wound around the central axis A.

In order to connect the superconductive coil 30 to an external powersource, a portion is drawn out from the superconductive wire 10 woundaround the central axis A (the portion drawn out from thesuperconductive wire 10 wound around the central axis

A is referred to as a lead-out portion 32). The minimum radius ofcurvature R_(min) of the lead-out portion 32 in the superconductive coil30 is, for example, 20 mm or less.

(Configuration of Superconductive Cable)

Hereinafter, the configuration of a superconductive cable 40 accordingto an embodiment will be described.

FIG. 15 is a side view illustrating the superconductive cable 40according to the embodiment. As illustrated in FIG. 15, thesuperconductive cable 40 includes a former 41 and a superconductive wire10. The superconductive wire 10 is spirally wound around the centralaxis of the former 41 on the outer peripheral surface of the former 41.

FIG. 16 is a cross-sectional view taken along line XVI-XVI of FIG. 15.As illustrated in FIG. 16, the minimum radius of curvature R_(m)in ofthe superconductive wire 10 is 20 mm or less after being wound on theouter peripheral surface of the former 41.

(Effects of Superconductive Wire, Stacked Superconductive Wire,Superconductive Coil, and Superconductive Cable)

Hereinafter, effects of the superconductive wire 10, the stackedsuperconductive wire 20, the superconductive coil 30, and thesuperconductive cable 40 according to the embodiment will be described.

<Effects of Superconductive Wire>

First, the basic effects of the superconductive wire 10 will bedescribed.

In the superconductive wire 10, the first superconductive layer 11 c iselectrically connected to the first substrate 11 a made of a conductivematerial via the first intermediate layer 11 b made of a conductivematerial. The first superconductive layer 11 c is further electricallyconnected to the second superconductive layer 12 c. Therefore, when aquench occurs in the first superconductive layer 11 c, the electriccurrent flowing in the first superconductive layer 11 c is bypassed tothe first substrate 11 a and the second superconductive layer 12 c.Similarly, when a quench occurs in the second superconductive layer 12c, the electric current flowing in the second superconductive layer 12 cis bypassed to the second substrate 12 a and the first superconductivelayer 11 c.

Thus, in the superconductive wire 10, when a quench occurs in the firstsuperconductive layer 11 c (or the second superconductive layer 12 c),the electric current flowing in the first superconductive layer 11 c (orthe second superconductive layer 12 c) is bypassed by two paths.Therefore, according to the superconductive wire 10, the anti-quenchresistance is improved.

In the superconductive wire 10, since the first member 11 and the secondmember 12 are stacked along the thickness direction so that the firstsuperconductive layer 11 c and the second superconductive layer 12 cface each other, the first superconductive layer 11 c and the secondsuperconductive layer 12 c are disposed relatively close to the neutralline of the superconductive wire 10.

The bending stress δ, the bending moment M, the cross-sectionalsecondary moment I, and the distance y from the neutral line satisfy therelationship of δ=(M/I)×y. In other words, the portion of thesuperconductive wire 10 which is relatively close to the neutral linehas a smaller value of y, and thereby, the bending stress acting thereonis smaller. Accordingly, it is possible to reduce the bending stressacting on the first superconductive layer 11 c and the secondsuperconductive layer 12 c in the superconductive wire 10, and as aresult, it is possible to prevent the first superconductive layer 11 cand the second superconductive layer 12 c from being damaged.

Next, an additional effect of the superconductive wire 10 will bedescribed.

In the case where the first superconductive layer 11 c and the secondsuperconductive layer 12 c are superconductively bonded, when a quenchoccurs in the first superconductive layer 11 c (or the secondsuperconductive layer 12 c), the electric current flowing in the firstsuperconductive layer 11 c (or the second superconductive layer 12 c)may be bypassed to the second superconductive layer 12 c (or the firstsuperconductive layer 11 c) without passing through the normal conductor(the first protective layer 11 d and the second protective layer 12 d).In addition, in this case, since the thickness of the superconductivelayer is substantially large, the local degradation of characteristicsis less likely to occur in the superconductive layers (the firstsuperconductive layer 11 c and the second superconductive layer 12 c).

In the case where the first protective layer 11 d and the secondprotective layer 12 d are made of silver or a silver alloy, since thefirst protective layer 11 d and the second protective layer 12 d arerelatively easily deformed, the first protective layer 11 d and thesecond protective layer 12 d may be directly bonded to each other byheating and pressing. Since silver or a silver alloy has a lowerelectric resistance than the material (typically, tin (Sn) alloy)constituting the bonding layer 13, it is possible to reduce the electricresistance when bypassing the electric current that flows in the firstsuperconductive layer 11 c (or the second superconductive layer 12 c) tothe second superconductive layer 12 c (or the first superconductivelayer 11 c).

In the case where the first protective layer 11 d and the secondprotective layer 12 d are made of silver or a silver alloy, since silveror a silver alloy may be satisfactorily bonded to the material(typically, a tin alloy) of the bonding layer 13, it is possible tosatisfactorily bond the first protective layer 11 d and the secondprotective layer 12 d via the bonding layer 13.

As described above, the first protective layer 11 d (or the secondprotective layer 12 d) is only used to bypass the electric current tothe second superconductive layer 12 c (or the first superconductivelayer 11 c) when a quench occurs in the first superconductive layer 11 c(or the second superconductive layer 12 c). Therefore, even if the firstprotective layer 11 d and the second protective layer 12 d are formed tobe relatively thin, the influence on the anti-quench resistance issmall. On the other hand, since the first protective layer 11 d and thesecond protective layer 12 d may be formed to be relatively thin, thefirst superconductive layer 11 c and the second superconductive layer 12c may be arranged closer to the neutral line of the superconductive wire10, which makes it possible to further reduce the bending stress actingon the first superconductive layer 11 c and the second superconductivelayer 12 c when the superconductive wire 10 is being bent.

As described above, when a quench occurs in the first superconductivelayer 11 c (or the second superconductive layer 12 c), the electriccurrent flowing in the first superconductive layer 11 c (or the secondsuperconductive layer 12 c) is also bypassed to the first substrate 11 a(or the second substrate 12 a). In the case where the first substrate 11a includes the first conductive layer 11 ab (or in the case where thesecond substrate 12 a includes the second conductive layer 12 ab), thebypassed current flows through the first conductive layer 11 ab (or thesecond conductive layer 12 ab) having a relatively low electricresistance, which makes it possible to further reduce the electricresistance when bypassing the electric current.

In the case where at least one of the first conductive layer 11 ab andthe second conductive layer 12 ab is exposed at the first end 10 a(and/or the second end 10 b ) by removing at least one of the first baselayer 11 aa and the second base layer 12 aa therefrom, it is possible toconnect the superconductive wire to an external power source via thefirst conductive layer 11 ab or the second conductive layer 12 ab havinga relatively low electric resistance.

In the case where both the first conductive layer 11 ab and the secondconductive layer 12 ab are exposed at the first end 10 a (and/or thesecond end 10 b ) by removing both the first base layer 11 aa and thesecond base layer 12 aa therefrom, it is possible to connect thesuperconductive wire to an external power source via both the firstconductive layer 11 ab and the second conductive layer 12 ab having arelatively low electric resistance, which makes it possible to furtherreduce the connection resistance to the external power supply.

In the case where only one of the first conductive layer 11 ab and thesecond conductive layer 12 ab is exposed at the first end 10 a (and/orthe second end 10 b ) by removing only one of the first base layer 11 aaand the second base layer 12 aa therefrom, it is possible to connect thesuperconductive wire to an external power source via the firstconductive layer 11 ab (or the second conductive layer 12 ab) having arelatively low electric resistance while maintaining the rigidity of thefirst end 10 a (and/or the second end 10 b ) via the second base layer12 aa (or the first base layer 11 aa). As a result, the handling of thesuperconductive wire 10 is improved.

<Effects of Stacked Superconductive Wire>

A conventional superconductive wire may have a width which is severaltens of times larger than its thickness. Such a superconductive wire isdifficult to be handled due to the large dimension. Since the stackedsuperconductive wire 20 is obtained by stacking a plurality ofsuperconductive wires 10 in the thickness direction, and therebysatisfies the relationship of 0.5≤T4/W≤2.0, the handling of the stackedsuperconductive wire 20 is improved.

<Effects of Superconductive Coil>

If the superconductive wire 10 is impregnated with the insulatingmaterial 31 while being wound around the central axis A, due to thedifference in thermal expansion coefficient between the superconductivewire 10 and the insulating material 31, a tensile stress may act on thesuperconductive wire in a direction of peeling off the superconductivelayer. This tensile stress may peel the superconductive layer off fromthe superconductive wire.

However, since the first superconductive layer 11 c and the secondsuperconductive layer 12 c in the superconductive wire 10 are sandwichedbetween the first substrate 11 a and the second substrate 12 a, thetensile stress acting on the first superconductive layer 11 c and thesecond superconductive layer 12 c is reduced. Therefore, according tothe superconductive coil 30, it is possible to prevent the firstsuperconductive layer 11 c and the second superconductive layer 12 cfrom being peeled off by the tensile stress caused by the difference inthermal expansion coefficient between the superconductive wire 10 andthe insulating material 31.

In the superconductive coil 30, the radius of curvature of the lead-outportion 32 may be made relatively small. In the superconductive wire 10,since the first superconductive layer 11 c and the secondsuperconductive layer 12 c are arranged close to the neutral line of thesuperconductive wire 10, the bending stress acting on the firstsuperconductive layer 11 c and the second superconductive layer 12 cwhen the superconductive wire 10 is being bent may be reduced.Therefore, it is possible to form the superconductive coil 30 in whichthe lead-out portion 32 has a minimum radius of curvature R_(min) of 20mm or less by using the superconductive wire 10.

<Effects of Superconductive Cable>

As described above, in the superconductive wire 10, the bending stressacting on the first superconductive layer 11 c and the secondsuperconductive layer 12 c when the superconductive wire 10 is beingbent may be reduced. Therefore, it is possible to form thesuperconductive cable 40 which has a minimum radius of curvature R_(min)of 20 mm or less by using the superconductive wire 10.

It should be understood that the embodiments disclosed herein have beenpresented for the purpose of illustration and description but notlimited in all aspects. It is intended that the scope of the presentdisclosure is not limited to the description above but defined by thescope of the claims and encompasses all modifications equivalent inmeaning and scope to the claims.

REFERENCE SIGNS LIST

10: superconductive wire; 10 a: first end; 10 b: second end; 11: firstmember; 11 a: first substrate; 11 aa: first base layer; 11 ab: firstconductive layer; 11 b: first intermediate layer; 11 c: firstsuperconductive layer; 11 d: first protective layer; 12: second member;12 a: second substrate; 12 aa: second base layer; 12 ab: secondconductive layer; 12 b: second intermediate layer; 12 c: secondsuperconductive layer; 12 d: second protective layer; 13: bonding layer;14: bonding layer; 15: lead; R_(min): minimum radius of curvature; T1,T2, T3, T3, T4: thickness; W: width; 20: stacked superconductive wire;30: superconductive coil; 31: insulating material; 32: lead-out portion;40: superconductive cable; 41: former

1. A superconductive wire comprising: a first member; and a secondmember, the first member including a first substrate made of aconductive material, a first intermediate layer made of a conductivematerial and disposed on the first substrate, and a firstsuperconductive layer made of a superconductive material and disposed onthe first intermediate layer, the second member including a secondsubstrate made of a conductive material, a second intermediate layermade of a conductive material and disposed on the second substrate, anda second superconductive layer made of a superconductive material anddisposed on the second intermediate layer, the first member and thesecond member being stacked along a thickness direction of thesuperconductive wire so that the first superconductive layer and thesecond superconductive layer face each other, and the firstsuperconductive layer being electrically connected to the secondsuperconductive layer.
 2. The superconductive wire according to claim 1,wherein the first superconductive layer is superconductively bonded tothe second superconductive layer.
 3. The superconductive wire accordingto claim 1, wherein the first member further includes a first protectivelayer made of silver or a silver alloy and disposed on the firstsuperconductive layer, the second member further includes a secondprotective layer made of silver or a silver alloy and disposed on thesecond superconductive layer, and the first protective layer is directlybonded to the second protective layer.
 4. The superconductive wireaccording to claim 1, wherein the superconductive wire further includesa bonding layer, the first member further includes a first protectivelayer made of a conductive material and disposed on the firstsuperconductive layer, the second member further includes a secondprotective layer made of a conductive material and disposed on thesecond superconductive layer, and the first protective layer is bondedto the second protective layer via the bonding layer.
 5. Thesuperconductive wire according to claim 4, wherein the first protectivelayer and the second protective layer are made of silver or a silveralloy, and the bonding layer is made of solder.
 6. The superconductivewire according to claim 4, wherein the bonding layer is exposed at anend of the superconductive wire in the longitudinal direction byremoving either the first member or the second member therefrom.
 7. Thesuperconductive wire according to claim 4, wherein the first protectivelayer and the second protective layer each has a thickness of 5 μm orless.
 8. The superconductive wire according to claim 1, wherein thefirst substrate includes a first base layer, and a first conductivelayer made of a material having a lower electric resistance than amaterial of the first base layer and disposed between the first baselayer and the first intermediate layer, the second substrate includes asecond base layer, and a second conductive layer made of a materialhaving a lower electric resistance than a material of the second baselayer and disposed between the second base layer and the secondintermediate layer.
 9. The superconductive wire according to claim 8,wherein the first conductive layer is exposed at an end of thesuperconductive wire in the longitudinal direction by removing the firstbase layer therefrom.
 10. The superconductive wire according to claim 8,wherein the second conductive layer is exposed at an end of thesuperconductive wire in the longitudinal direction by removing thesecond base layer therefrom.
 11. The superconductive wire according toclaim 8, wherein the first conductive layer and the second conductivelayer are made of copper or a copper alloy, and the first substrate andthe second substrate are made of stainless steel or Hastelloy.
 12. Thesuperconductive wire according to claim 1, wherein the first member andthe second member are spaced from each other at an end of thesuperconductive wire in the longitudinal direction.
 13. A stackedsuperconductive wire comprising a plurality of superconductive wiresaccording to claim 1, the plurality of superconductive wires beingstacked along a thickness direction of the stacked superconductive wire,and a value obtained by dividing a thickness of the stackedsuperconductive wire by a width of the stacked superconductive wire in adirection orthogonal to the longitudinal direction of the stackedsuperconductive wire being 0.5 or more and 2.0 or less.
 14. Asuperconductive coil comprising: the superconductive wire according toclaim 1; and an insulating material, the superconductive wire beingwound around a central axis of the superconductive coil and impregnatedwith the insulating material.
 15. The superconductive coil according toclaim 14, wherein the superconductive wire has a lead-out portion drawnout from the superconductive wire wound around the central axis of thesuperconductive coil, and a minimum radius of curvature of the lead-outportion in the superconductive wire is 20 mm or less.
 16. Asuperconductive cable comprising: the superconductive wire according toclaim 1; and a former, the superconductive wire being spirally woundaround a central axis of the former on an outer peripheral surface ofthe former, and a minimum radius of curvature of the superconductivewire being 20 mm or less.